Exploring, drilling and completing hydrocarbon and other wells are generally complicated, time consuming and ultimately very expensive endeavors. In recognition of the potentially enormous expense of well completion, added emphasis has been placed on well monitoring and maintenance throughout the life of the well. By the same token, added emphasis may be placed on materials used in the construction of downhole tools, equipment, tubulars and other devices in light of the harsh downhole environment. All in all, such added emphasis may increase the life of such equipment, if not the life and productivity of the well itself. As a result, this may help ensure that the well provides a healthy return on the significant investment involved in its completion.
The introduction of downhole devices such as the above noted tools, equipment, and tubulars is standard practice throughout well completion and production operations. In many cases, such as with production tubing, the devices are left disposed within the well for extended periods of time, such as for the useful life of the well. Depending on the hydrocarbon reservoir itself and the parameters of the operation, such durations may exceed several years.
Unfortunately, devices such as production tubing may include components susceptible to damage upon exposure to the downhole conditions of the well. Namely, stainless steel or other metals and alloys which constitute the main body of such devices are particularly prone to corrosion and environmental cracking upon prolonged exposure to downhole well conditions. For example, water cut, chemical makeup, and pressure or temperature extremes of the downhole environment may tend to induce corrosion and cracking in exposed metal and alloys. Indeed, corrosives such as hydrogen sulfide, halides, chloride, and carbon dioxide, common in most hydrocarbon wells, generally play a substantial role in corrosion and cracking of downhole devices and limiting the useful life of such exposed devices.
In order to address the noted cracking issue, alternative materials may be utilized to make up the main body structure of downhole devices. For example, any number of austenitic nickel-chromium-based superalloys may be utilized in constructing a downhole tubular such as the above noted production tubing. Such superalloys are particularly resistant to corrosion and cracking upon exposure to the harsh chemical environment common to hydrocarbon wells.
Unfortunately, it is cost prohibitive to employ such superalloys on all downhole devices. Indeed, constructing the noted production tubing of a nickel-chromium-based superalloy, would be so expensive that it would ultimately be far cheaper to complete the well, produce through stainless production tubing, and replace and repair the corroded tubing over time. Such prolonged maintenance may run several hundred thousand dollars and yet fail to completely keep the deteriorating tubing in a usable condition. Ultimately, the tubing may be replaced as noted or the well prematurely shut down at a significant cost in terms of lost production.
In light of the issues noted above, efforts have been made to improve corrosion crack resistance for less expensive materials such as stainless steel. For example, downhole device parts are often subjected to conventional shot peening. Similar to a small scale sand blasting technique, shot peening is a technique whereby ceramics or other heavy particles, significantly less than about 2 mm in size, are directed with substantial velocity at device parts. As such, a compressive layer is formed at the surfaces of such parts leaving them less susceptible to corrosion cracking.
Unfortunately, while shot peening is effective in extending the life of downhole device parts, the effect is limited. For example, the achievable thickness of the compressive layer is practically limited to less than about a micron due to the tendency of grain dislocations to effect material recovery in the face of shot peening. Furthermore, devices such as the above noted tubulars do not readily lend themselves to shot peening. For example, it may be beneficial to treat both inner and outer diameter surfaces of production tubing. However, treating the inner surface of such tubing is not available via shot peening. Thus, as a practical matter, shot peening treatments are generally limited to drill collars, testing tools, ball valves, and other discrete parts. Further, even where employed, the effectiveness of shot peening remains limited due to the noted limitations on compressive layer thicknesses.